interim patch for get_user_vec3d_value

As described in #3079, the old parsing of fvec and uvec used strtod and some POFs rely on this format.  This is a quickfix that uses the new parsing method for only `get_user_vec3d_value`.

Author: Goober5000

code/model/modelread.cpp

@@ -462,6 +462,55 @@ void get_user_prop_value(char *buf, char *value)
 	*p1 = c;
 }
 
+//	Stuff a floating point value pointed at by Mp.
+//	Advances past float characters.
+int stuff_float_temp(float *f, bool optional)
+{
+	char *str_start = Mp;
+	char *str_end;
+
+	// since strtof ignores white space anyway, might as well make it explicit
+	ignore_white_space();
+
+	auto result = strtof(Mp, &str_end);
+	bool success = false, comma = false;
+	int retval = 0;
+
+	// no float found?
+	if (result == 0.0f && str_end == Mp)
+	{
+		if (!optional)
+			error_display(1, "Expecting float, found [%.32s].\n", next_tokens());
+	}
+	else
+	{
+		*f = result;
+		success = true;
+	}
+
+	if (success)
+		Mp = str_end;
+
+	if (*Mp == ',')
+	{
+		comma = true;
+		Mp++;
+	}
+
+	if (optional && !success)
+		Mp = str_start;
+
+	if (success)
+		retval = 2;
+	else if (optional)
+		retval = comma ? 1 : 0;
+	else
+		skip_token();
+
+	diag_printf("Stuffed float: %f\n", *f);
+	return retval;
+}
+
 // routine to parse out a vec3d from a user property field of an object
 bool get_user_vec3d_value(char *buf, vec3d *value, bool require_brackets)
 {
@@ -494,11 +543,11 @@ bool get_user_vec3d_value(char *buf, vec3d *value, bool require_brackets)
 		}
 
 		// get comma-separated floats
-		if (stuff_float_optional(&f1) != 2)
+		if (stuff_float_temp(&f1, true) != 2)
 			break;
-		if (stuff_float_optional(&f2) != 2)
+		if (stuff_float_temp(&f2, true) != 2)
 			break;
-		if (stuff_float_optional(&f3) != 2)
+		if (stuff_float_temp(&f3, true) != 2)
 			break;
 
 		if (require_brackets)